The present disclosure relates to a plasma torch. The invention finds particular use in the abatement of exhaust gases from processes, such as those from the semiconductor industry.
Preventing or limiting the emission of hazardous gases exhausted from industrial processes to the atmosphere is now a major focus of both the scientific and industrial sectors. In particular the semiconductor industry, where the use of process gases is inherently inefficient, has set its own targets for reducing the amount of gases exhausted to the atmosphere from fabrication plants. Examples of compounds which it is desirable to destroy are those from etch processes such as fluorine, SF6, NF3 or perfluorocarbons (CF4, C2F6 etc.)
One method of destroying, or abating, unwanted gases from an exhaust gas stream uses a plasma abatement device. Plasmas are particularly useful when the fuel gases normally used for abatement by combustion are not readily available; for example, as described in EP1773474.
Plasmas for abatement devices can be formed in a variety of ways. Microwave plasma abatement systems can be connected to the exhaust of several process chambers. However, each device requires its own microwave generator which can add considerable cost to a system. DC plasma torch abatement devices are advantageous over microwave plasma devices in that a plurality of torches may be operated from a single power DC power supply.
An example of a known DC plasma torch is shown schematically, in cross-section, in FIG. 1. The torch 10 comprises a generally cylindrical cathode 12 partially nested within an upstream opening of a generally tubular anode 14. An annular space 16 is provided between the cathode 12 and anode 14, through which a plasma source gas such as argon or nitrogen (not shown) can flow.
The cathode 12, and optionally the anode 14, is electrically connected to a power supply (not shown), which can be configured to apply a DC voltage between the cathode 12 and anode 14, or an AC voltage to either or both of the cathode 12 and anode 14. The magnitude and frequency of the voltage required is generally determined and selected by reference other process parameters, such as the exhaust gas or plasma source gas species and flow rate, the cathode-anode spacing, gas temperature etc. In any event, an appropriate voltage regime is one that causes the gas to ionise and thereby form a plasma.
In the illustrated prior art example of FIG. 1, it will be noted that the interior geometry of the tubular anode 14 comprises (going from the upstream end (shown uppermost in the drawing) to the downstream end (shown lowermost in the drawing)) a first inwardly-tapering frusto-conical portion 18 leading to a substantially parallel-sided throat portion 20, which leads to an outwardly-tapering frusto-conical portion 22. The effect of this geometry is to accelerate and compress incoming gas to create a small region 24 of relative high speed, relatively compressed gas in a region immediately downstream of the cathode 12,
The cathode 12 comprises a generally cylindrical body portion 26 leading to a chamfered free end portion 28 whose external geometry substantially matches the internal geometry of the inwardly-tapering frusto-conical portion 18 of the anode 14. The body portion 26 of the cathode 12 is manufactured from a high-conductivity metal, such as copper, which is usually water-cooled. At the centre of the generally planar lower face 30 of the cathode 12, there is provided an axially-projecting button-type cathode 32, which provides a preferential electrical discharge site. This is accomplished by selecting a different material for the button 32 than the main body 28 of the cathode arrangement, i.e. such that the cathode body 28 is formed of a conducting metal with a higher thermal conductivity and work function than that of the thermionic material of the button cathode 32. For example it is common to use a copper cathode body 28 and a hafnium button 32. The anode 14 can be formed of a similar material to the main body portion 28 of the cathode 12, e.g. copper
It will be noted that the button cathode 32 is positioned in the region of relative high speed, relatively compressed gas 24. The effect of such an arrangement is to create a region of preferential electrical discharge for the plasma source gas, when in a relatively compressed, high-speed, state; i.e. suitable for the formation of a plasma 34. The plasma 34 is thus nucleated in the region immediately below the cathode 12 and exits as a jet via the throat 20 and expands and decelerates thereafter in the outwardly-tapering frusto-conical portion 22 of the anode 14.
In operation of the plasma torch of FIG. 1, the plasma source, or feed, gas (i.e. a moderately inert ionisable gas such as nitrogen, oxygen, air or argon) is conveyed to the annular space 16 via an inlet manifold (not shown). To initiate, or start the plasma torch, a pilot arc must first be generated between the thermionic button cathode and the anode. This is achieved by a high frequency, high voltage signal, which may be provided by a generator associated with the power supply for the torch 10 (not shown). The difference in thermal conductivity between the copper body 26 and the hafnium button 32 of the cathode arrangement means that the cathode temperature will be higher and the electrons are preferentially emitted from the button 32. Therefore when the aforementioned signal is provided between the electrodes 12 and 14 a spark discharge is induced in the plasma source gas flowing into the plasma forming region 24. The spark forms a current path between the anode 14 and cathode 12; the plasma is then maintained by a controlled direct current between the anode 14 and the cathode 12. The plasma source gas passing through the exit throat 20 produces a high momentum plasma flare of ionised source gas.
In most cases, the plasma flare will be unstable and cause anode erosion, it therefore need to be stabilised by generating a spiral flow, or vortex, of the inlet plasma gas between the electrodes 12, 14.
One method of creating the vortex, or gas swirl, is by the use of a cathode arrangement which comprises a swirl bush element. An example of this type of known arrangement is shown in FIG. 2. For simplicity in identical features appearing in FIGS. 1 and 2 have been given the identical reference signs and will not be described again.
The cathode arrangement 12 as shown in FIG. 2 is substantially the same as that shown in FIG. 1, except that it additionally comprises an annular swirl bush 40. The swirl bush 40 is formed from a generally tubular element interposed between the cathode 12 and anode 14. Although not discernable from the drawings, the swirl bush 40 comprises a plurality of non-linear (e.g. part-helical) grooves or vanes that form non-axial flow channels for sub-streams of the gas.
The outer surface of the swirl bush 40 is formed to cooperate with a portion of the inwardly-tapering frusto-conical surface portion of the anode arrangement 14. The outer surface of the swirl bush 40 substantially matches the internal wall angle of the cooperating portion of the frusto-conical anode 12 and further comprises angular grooves in its surface which form conduits for guiding the flow of plasma source gas. The angular grooves may also, or instead, be formed in the surface of the cooperating portion of the frusto-conical anode 18.
The effect of the vanes or grooves is to cause discrete sub-streams of the gas to flow along spiralling trajectories thereby creating a vortex in the region of relative high speed, relatively compressed gas 24 where the individual sub-streams of gas converge. The rotational component of the gas' momentum as it exits via the throat 20 of the torch 10 causes the plasma jet 34 to self-stabilise.
In order for the torch 10 to function, the cathode 12 and anode 14 must be electrically isolated from one another. As such, any element interposed between, and in contact with both, the cathode 12 and anode 14 must be electrically insulating. In this case, the swirl bush 40 is manufactured of a dielectric material, 1 such as PTFE, which functions as an electrical insulator between the two electrodes 12, 14 and is also somewhat resistant to chemical attack by the high reactive plasma ions, such as atomic fluorine produced during the abatement of perfluorocarbons if they are passed through this region.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any or all disadvantages noted in the background.